Silicon nitride, Si.sub.3 N.sub.4, is recognized as a candidate material for high temperature-high performance applications which require the benefits of a ceramic material. Considerable attention has been paid to the preparation of Si.sub.3 N.sub.4 powders and sintering aids.
The covalent nature of Si.sub.3 N.sub.4 makes it very difficult to sinter and densify. Sintering aids are therefore utilized to prepare dense ceramic bodies. Sintering aids form a liquid phase at high temperatures and act as a medium for sintering Si.sub.3 N.sub.4 via a dissolution-precipitation process. The viscosity and melting point of the liquid phase must be suitable to allow sintering of Si.sub.3 N.sub.4 to occur. The resultant microstructure generally contains intergrown Si.sub.3 N.sub.4 crystals or grains, with glassy and/or crystalline phases located along grain boundaries and at grain junctions, which phases solidified from the liquid sintering phase. The grain boundary phase(s) are most commonly oxide, silicate or oxynitride compositions.
There is a great deal of literature and patent documentation on oxide and non-oxide additives capable of sintering Si.sub.3 N.sub.4. For example, U.S. Pat. No. 4,102,698 discloses the use of Y.sub.2 O.sub.3 and SiO.sub.2 to form Si.sub.3 N.sub.4 ceramics with good strength and oxidation resistance.
Japanese Application 86/297535 discloses the use of magnesium silicates with ZrO.sub.2 to sinter Si.sub.3 N.sub.4 at 1700.degree. C. Japanese Application 88/151679 utilizes MgAl.sub.2 O.sub.4 (magnesium aluminate) as a sintering aid for Si.sub.3 N.sub.4. The use of alkaline earth oxides or MgAl.sub.2 O.sub.4 to sinter Si.sub.3 N.sub.4 is disclosed in British Patent 1536290. Ceramic composites of Si.sub.3 N.sub.4 with oxides, such as cordierite (Mg.sub.2 Al.sub.5 Si.sub.5 O.sub.18), are claimed in U.S. Pat. No. 4,542,109.
The use of CaO--TiO.sub.2 --SiO.sub.2 glass solder for joining is discussed in two articles by N. Iwamoto, U. Norimasa and Y. Haibara; Trans. JWRI, 15(2), 265-71, (1986) and N. Iwamoto, U. Norimasa and Y. Haibara; J. Car. Soc. Japan, 94(8), 880-86, (1986). The resultant Ti nitride bond strongly secured two Si.sub.3 N.sub.4 pieces previously sintered using Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 additives.
Y.sub.2 O.sub.3 -containing sintering aids e.g., Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --SiO.sub.2, give good mechanical properties for ambient and elevated temperature applications, as well as high temperature oxidation resistance. However, yttria is expensive and yttria-containing aids generally require temperatures of at least 1700.degree.-1750.degree. C. to form dense Si.sub.3 N.sub.4 ceramics. Thus, there remains a need for less expensive, yet effective sintering aids for Si.sub.3 N.sub.4. Additionally, there is a need for a sintering aid which allows for extensive densification at lower sintering temperatures.